Process for the production of olefins

ABSTRACT

A process for the production of olefins from a hydrocarbon comprising the steps of: a) passing a first feed stream comprising gaseous reactants to a first reaction zone wherein said gaseous reactants react exothermically to provide a product stream b) producing a mixed feed stream comprising oxygen by passing the product stream produced in step (a) and a second feed stream comprising a hydrocarbon feedstock to a mixing zone, oxygen being passed to the mixing zone via (i) the product stream produced in step (a), (ii) the second feed stream comprising a hydrocarbon feedstock and/or (iii) a third stream comprising an oxygen-containing gas c) passing the mixed feed stream directly to an essentially adiabatic second reaction zone wherein in the absence of a supported platinum group metal catalyst at least a part of the oxygen is consumed and a stream comprising olefins is produced e) cooling the stream comprising olefins exiting the second reaction zone to less than 650° C. within less than 150 milliseconds of formation and wherein the temperature of the mixed stream is at least 500° C., the mixing zone and the second reaction zone are maintained at a pressure of between 1.5-50 bar and the residence time within the mixing zone is less than the autoignition delay for the mixed stream.

This application is the U.S. National Phase of International ApplicationPCT/GB2003/004993, filed 18 Nov. 2003, which designated the U.S.PCT/GB2003/004993 claims priority to British Application No. 0229497.3filed 18 Dec. 2002. The entire content of these applications areincorporated herein by reference.

The present invention relates to a process for the production ofolefins.

Olefins such as ethylene and propylene may be produced by the catalyticdehydrogenation or cracking of a hydrocarbon feed. In this applicationthe term “cracking” will be used to embrace both these chemicalreactions. In an auto-thermal cracking process, a hydrocarbon feed ismixed with an oxygen-containing gas and contacted with a catalystcapable of supporting combustion beyond the fuel rich limit offlammability. The hydrocarbon feed is partially combusted and the heatproduced is used to drive the cracking reaction.

An example of an auto-thermal cracking process is described in EP 0 332289. The document describes the use of a paraffinic feed of, forexample, ethane, propane and/or butane which is mixed with oxygen andcracked to produce an olefinic mixture. The cracking reaction isendothermic and is carried out at elevated temperatures.

The energy required for the cracking reaction is provided by combustionof a part of the feed and the feed may also be preheated but thetemperature is limited due to the risk of autoignition.

Alternatively the energy required may be provided by a preliminaryheat-generating step. In this step a gaseous fuel reacts with oxygen inan exothermic reaction in the presence of a catalyst. The reactionconditions are controlled to ensure that not all of the oxygen isconsumed during this process. The thermal energy produced by thereaction heats the unreacted oxygen, thereby providing an additionalsource of heat to drive the cracking of the hydrocarbon feedstock. Thisprocess is described in EP 1 112 241.

However using a catalyst to promote the autothermal cracking process canbe problematic because the catalyst is prone to deactivate andconsequently needs to be periodically replaced or reactivated. Thecatalyst may also provide a variable performance. Furthermore thecatalyst may cause blockages in the reactor system.

It has now been found that an autothennal cracking process can becarried out in the substantial absence of a catalyst.

According to the present invention there is provided a process for theproduction of olefins from a hydrocarbon said process comprising thesteps of:

-   a) passing a first feed stream comprising gaseous reactants to a    first reaction zone wherein said gaseous reactants react    exothermically to provide a product stream-   b) producing a mixed feed stream comprising oxygen by passing the    product stream produced in step (a) and a second feed stream    comprising a hydrocarbon feedstock to a mixing zone and wherein    oxygen is passed to the mixing zone via one or more of (i) the    product stream produced in step (a), (ii) the second feed stream    comprising a hydrocarbon feedstock and (iii) a third stream    comprising an oxygen-containing gas-   c) passing the mixed feed stream directly to an essentially    adiabatic second reaction zone wherein in the absence of a supported    platinum group metal catalyst at least a part of the oxygen is    consumed and a stream comprising olefins is produced-   d) cooling the stream comprising olefins exiting the second reaction    zone to less than 650° C. within less than 150 milliseconds of    formation

and wherein the temperature of the mixed stream is at least 500° C., themixing zone and the second reaction zone are maintained at a pressure ofbetween 1.5-50 bar and the residence time within the mixing zone is lessthan the autoignition delay for the mixed stream.

Additional feed streams comprising at least one from carbon monoxide,carbon dioxide, steam and hydrogen may also be passed to the mixingzone.

Preferably an additional feed stream comprising hydrogen is passed tothe mixing zone.

The gaseous reactants in the first feed stream may be any reactants thatare capable of reacting exothermically. The heat generated from theexothermic reaction is transferred to the mixed stream via the resultantproduct stream produced in step (a).

The gaseous reactants may comprise a gaseous fuel and anoxygen-containing gas.

The gaseous fuel may be any gaseous fuel which is capable of reactingwith oxygen in an exothermic reaction. Suitable examples includehydrocarbons, such as methane, ethane, propane and butane. Methane isthe preferred gaseous fuel. Other suitable fuels include hydrogen,carbon monoxide, alcohols (e.g. methanol, ethanol), oxygenates and/orammonia. Waste fuel streams may also be employed.

The oxygen-containing gas may comprise air, oxygen and/or an air/oxygenmixture. The oxygen-containing gas may be mixed with an inert gas suchas nitrogen, helium or argon.

The first reaction zone may contain an ignition source such as a pilotflame or a spark ignition source which is used to initiate theexothermic reaction. Alternatively the first reaction zone may comprisea catalyst.

Wherein the first reaction zone contains a catalyst the catalyst usuallycomprises an oxidation catalyst such as a supported platinum groupmetal. Suitable catalyst supports include a range of ceramic and metalsupports, with alumina supports being preferred. The support may be inthe form of spheres or other granular shapes and may be present as athin layer or wash coat on another substrate. Preferably the substrateis a continuous multi-channel ceramic structure such as a foam or aregular channelled monolith. In a preferred embodiment, the supportcomprises a gamma-alumina coated alpha-alumina. Alternatively zirconiaor a gamma-alumina coated lithium aluminium silicate foam support may beemployed.

The first feed stream may be passed to the first reaction zone at atemperature of up to 800° C., preferably between 200 and 600° C. andreacted at at temperature between 600 and 1400° C., preferably between700 and 1200° C. and most preferably between 950 and 1100° C.

The first reaction zone may be maintained at any suitable pressure e.g.atmospheric pressure. Usually the first reaction zone is maintained at apressure of from 1.5 to 50 bara (bar absolute), for example between 1.8to 50 bara, preferably between 5-50 bara, most preferably between 5 to30 bara and advantageously between 10-30 bara. It will be understoodthat the precise pressures employed will vary depending on the specificreaction conditions and gaseous reactants employed.

The first feed stream is usually introduced into the first reaction zoneat a gas hourly space velocity (GHSV) of greater than 10,000 h⁻¹,preferably above 100,000 h⁻¹ and most preferably greater than 300,000h⁻. It will be understood that the optimum gas hourly space timevelocity will depend upon the pressure and nature of the feedcomposition.

In one embodiment of the invention the product stream produced in step(a) may comprise oxygen. Consequently the first feed stream may comprisea gaseous fuel and an oxygen-containing gas which may be passed to thefirst reaction zone wherein a product stream comprising unreacted oxygenis produced. The product stream may then be passed to the mixing zone toprovide the mixed feed stream comprising oxygen.

When the product stream comprising unreacted oxygen is produced thefirst feed stream comprising a gaseous fuel and an oxygen containing gasis preferably fuel-rich with a fuel to oxygen ratio above thestoichiometric ratio required for complete combustion. For example, thefuel to oxygen ratio in the feed may be 1.5 to 4 times, preferably 3times the stoichiometric ratio required for complete combustion tocarbon dioxide and water.

The gaseous fuel and oxygen-containing gas may be contacted in the firstreaction zone under reaction conditions which are controlled to ensurethat some of the oxygen in the first feed stream remains unreacted. Thethermal energy produced in step (a) heats the unreacted oxygen therebyproviding part of the heat necessary for cracking the hydrocarbonfeedstock in step (c).

The reaction between the gaseous fuel and oxygen-containing gas may be acombustion reaction. Accordingly, gaseous fuel in the first feed streammay react with oxygen to produce a product stream comprising oxides(e.g. carbon oxides) and water. In such an embodiment and wherein thefirst reaction zone contains a catalyst, a combustion catalyst isemployed. Suitable combustion catalysts include Group VIII metals suchas platinum and/or palladium. The catalyst may comprise 0.1 to 5 wt %and preferably 0.25 to 3 wt % of metal. It will be understood that themetal loadings of the catalyst may be selected to ensure that not allthe oxygen in the first feed stream is consumed in step (a).

In an alternative embodiment the gaseous fuel of the first feed streamreacts with the oxygen-containing gas to produce synthesis gas. In thisembodiment a first feed stream comprising a hydrocarbon (e.g. methane)is employed which reacts with oxygen to produce carbon monoxide andhydrogen. These gaseous products may react exothermically, for examplewith oxygen, thereby providing a further source of heat to drive thecracking reaction in step (c). In this embodiment, and wherein acatalyst is employed the catalyst is one which is capable of supportinga synthesis gas production reaction. Suitable catalysts compriserhodium, platinum, palladium, nickel or mixtures thereof. Preferably arhodium catalyst is used. The catalyst may comprise 0.1 to 5 wt % andpreferably 0.25 to 3 wt %, of metal. As with combustion catalysts, themetal loadings of the catalyst may be varied to ensure that not all theoxygen in the first feed stream is consumed in step (a).

In a further embodiment, a gaseous fuel is reacted with anoxygen-containing gas in a combustion reaction and another gaseous fuel(which may or may not be the same as the first gaseous fuel) is reactedwith an oxygen-containing gas to produce synthesis gas. Both thesereactions are exothermic and may provide part of the heat for drivingthe subsequent cracking reaction in step (c). In at least one of thesereactions, however, not all of the oxygen-containing gas employed isconsumed. At least part of this unreacted oxygen is consumed in step (c)to produce the olefin product of the present invention.

The product stream produced from step (a) is usually passed to themixing zone at a temperature of between 900 and 1400° C., preferablybetween 950 and 1250° C., and most preferably between 1000 and 1200° C.

In another embodiment of the invention the second feed stream maycomprise oxygen. The second feed stream comprising oxygen may then bemixed with the product stream produced in step (a) to provide the mixedfeed stream comprising oxygen.

The second feed stream may comprise any suitable hydrocarbon and mayoptionally comprise an oxygen containing gas. For example, gaseoushydrocarbons, heavy hydrocarbons or mixtures thereof may be employed.Suitable gaseous hydrocarbons include ethane, propane, butane andmixtures thereof. Suitable heavy hydrocarbons include naphtha, gas oil,vacuum gas oil, refinery residues, atmospheric residues, vacuum residuesand crude and fuel oils. Additional feed components such as hydrogen,nitrogen, carbon monoxide, carbon dioxide and steam may also be includedin the second feed stream. Hydrogen and/or carbon monoxide may reactwith the oxygen present to produce additional heat for driving thecracking process. In the second feed stream, a gaseous hydrocarbon maybe alternated with a heavy hydrocarbon, as the hydrocarbon feed stock.

The second feed stream is usually heated to a temperature of 200° C. to600° C., and preferably to 300° C. to 500° C.

The second feed stream is introduced at a gas hourly space velocity ofgreater than 10,000 h⁻¹, preferably above 20,000 h⁻¹ and most preferablygreater than 100,000 h⁻¹. It will be understood, however, that theoptimum gas hourly space time velocity will depend upon the pressure andnature of the feed composition.

The second feed stream comprising a hydrocarbon feedstock and optionallyoxygen is contacted with the product stream produced in step (a), whichmay or may not contain unreacted oxygen, in a mixing zone wherein amixed feed stream comprising oxygen is produced.

In yet another embodiment of the invention a third feed streamcomprising an oxygen-containing gas may be fed to the mixing zone toprovide the mixed feed stream comprising oxygen. The oxygen-containinggas may be any oxygen-containing gas as herein described above.

The temperature of the mixed feed stream is at least 500° C., preferablyat least 600° C. and most preferably at least 700° C.

The mixing zone is maintained at a pressure of between 1.5 to 50 bara,for example between 1.8 to 50 bara, preferably between 5-50 bara, mostpreferably between 5 to 30 bara and advantageously between 10-30 bara.

The mixing zone is usually a mixing channel that passes directly intothe second reaction zone. The residence time of the mixed feed stream inthe mixing zone is less than the autoignition delay of the mixed feedstream. Methods of determining the autoignition delay are known in theart. For example, ASTM 659-78(2000) is the Standard Test Method forMinimum Autoignition Temperature of Liquid Chemicals and may be easilyadapted to determine autoignition delay. A description of a suitablyadapted method is provided in the article “Spontaneous Ignition ofMethane: Measurement and Chemical Model” by I A B Reid, C Robinson and DB Smith, 20th Symposium (International) on Combustion/The CombustionInstitute 1984/pp 1833-1843. Under some conditions of temperature andpressure it may be difficult to measure the autoignition delay directly,but the delay can be predicted using data obtained under otherconditions, e.g lower temperature. Having a residence time in the mixingzone of less than the autoignition delay of the mixed feed streamensures that the production of olefins does not commence until the mixedfeed stream enters the second reaction zone. This means that there islittle consumption of oxygen in the mixing zone, typically less than 5%wt of the oxygen fed to the mixing zone is consumed in the mixing zone.

Typically the residence time within the mixing zone is less than 100milliseconds, preferably less than 50 milliseconds, most preferably lessthan 10 milliseconds and advantageously less than 5 milliseconds.

Preferably the mixed feed stream is passed through the mixing zone andinto the into the second reaction zone at a velocity of greater than 1m/s, preferably greater than 3 m/s. These velocities are sufficientlyhigh to prevent flashback into the first reaction zone.

The second reaction zone is essentially adiabatic so that reactiontherein occurs without significant amounts of heat entering or leavingthe reaction zone. This is achieved by insulating the second reactionzone.

The hydrocarbon feed in the second feed stream may be cracked intoolefins such as ethene, propene, butene and pentene or a mixturethereof.

The cracking reaction may be suitably carried out at a temperature ofbetween 600 and 1200° C., preferably between 850 and 1050° C. and mostpreferably, between 900 and 1000° C. It will be understood that theoptimum temperature will depend upon the feed mixture and operatingpressure.

The cracking reaction is carried out in the second reaction zone at apressure of between 1.5 to 50 bara, for example, between 1.8 to 50 bara,preferably between 5-50 bara, most preferably between 5 to 30 bara andadvantageously between 10-30 bara. It will be understood that theprecise pressures employed will vary depending on the specific reactionconditions and gaseous fuels employed. The use of higher pressures mayprovide improved stability and may enable a smaller reactor to beemployed. Generally, the use of a higher pressure would be used withlower temperatures and higher temperatures with lower pressures.

The second reaction zone does not contain a supported platinum groupmetal catalyst i.e. a catalyst comprising a metal of Group 10 of thePeriodic Table, particularly catalysts comprising platinum, palladium ormixtures thereof. Preferably the second reaction zone does not containany catalytic material that is capable of supporting combustion beyondthe normal fuel rich limit of flammability. Most preferably the secondreaction zone does not contain any material that would exhibit anysubstantial catalytic activity.

The second reaction zone may contain a stabiliser and/or packingmaterials such porcelain, ceramics, alumina and/or silica that do notexhibit any substantial catalytic activity.

In a preferred embodiment the second reaction zone may contain agrid(s), a perforated plate(s), and/or a baffle plate(s).

In another preferred embodiment of the invention the second reactionvessel may comprises an ignition source such as, for example, a heatedgauze which encourages the reaction to occur at a particular locationwithin the vessel.

In a further embodiment the second reaction zone is essentially empty.

In a preferred embodiment of the invention the residence time within thesecond reaction zone is sufficient to ensure that substantially all theoxygen is consumed.

Typically the residence time within the second reaction zone is lessthan 100 milliseconds, preferably less than 50 milliseconds andadvantageously less than 10 milliseconds.

After the cracking reaction the products are quenched as they emergefrom the second reaction zone such that the temperature is reduced toless than 650° C. within less than 150 milliseconds of formation. Theformation of the reaction products, particularly olefins, is assumed tobegin at the end of the mixing zone and so the quench time is the timebetween the reactants leaving the mixing zone and the reduction intemperature of the products to below 650° C.

Wherein the pressure of the second reaction zone is maintained at apressure of between 1.5-2.0 bara usually the products are quenched andthe temperature reduced to less than 650° C. within 100-150 millisecondsof formation.

Wherein the pressure of the second reaction zone is maintained at apressure of between 2.0-5.0 bara usually the products are quenched andthe temperature reduced to less than 650° C. within 50-100 millisecondsof formation.

Wherein the pressure of the second reaction zone is maintained at apressure of between 5.0-10.0 bara usually the products are quenched andthe temperature reduced to less than 650° C. within less than 50milliseconds of formation.

Wherein the pressure of the second reaction zone is maintained at apressure of between 10.0-20.0 bara usually the products are quenched andthe temperature reduced to less than 650° C. within 20 milliseconds offormation.

Finally wherein the pressure of the second reaction zone is maintainedat a pressure of greater than 20.0 bara usually the products arequenched and the temperature reduced to less than 650° C. within 10milliseconds of formation.

This avoids further reactions taking place and maintains a high olefinselectivity.

The products may be quenched using rapid heat exchangers of the typefamiliar in steam cracking technology. Additionally or alternatively, adirect quench may be employed. Suitable quenching fluids include waterand hydrocarbons such as ethane or naphtha.

The present invention usually provides a percentage conversion ofhydrocarbon of greater than 40%, preferably greater than 50%, and mostpreferably greater than 60%.

Furthermore the present invention usually provides a selectivity towardsolefins of greater than 20%, preferably greater than 30%, and mostpreferably greater than 50%.

Any coke produced in the process of the present invention may be removedby mechanical means, or using one of the decoking methods described inEP 0 709 446, incorporated herein by reference.

EXAMPLE 1

A first feed stream comprising the gaseous reactants methane and oxygenwas passed to the first reaction zone comprising a promoted palladiumcatalyst at a temperature of 400° C. wherein gaseous reactantsexothermically reacted to provide a synthesis gas product stream. Thefirst reaction zone was maintained at a pressure of 1.8 bara.

The product stream comprising hydrogen, carbon monoxide, carbon dioxide,and water was passed to the mixing zone at a flow rate of 12.54 g/minwherein the hydrogen flow rate was 1.31 g/min, the carbon monoxide flowrate was 9.48 g/min, the carbon dioxide flow rate was 0.75 g/min and thewater vapour flow rate was 1.00 g/min. The temperature of the productstream was 1200° C.

A second feed stream comprising ethane and oxygen was passed to themixing zone at a flow rate of 24.97 g/min wherein the ethane flow ratewas 18.80 g/min and the oxygen flow rate was 6.17 g/min. The temperatureof the second feed stream was 450° C.

The residence time in mixing channel was less than 5 ms.

The mixed feed stream with a combined temperature of 610° C. was passeddirectly to the second reaction zone wherein in the absence of acatalyst the oxygen was consumed and the ethane was converted toethylene. The second reaction zone was also maintained at a pressure of1.8 bara.

The product stream exiting the second reaction zone at a temperature of770° C. was cooled using a water quench to 600° C. within less than 50milliseconds of formation.

The % conversion of ethane was measured a 68.1% and the selectivitytowards ethylene was measured at 77.2%.

EXAMPLE 2

A first feed stream comprising the gaseous reactants methane and oxygenwas passed to the first reaction zone comprising a promoted palladiumcatalyst at a temperature of300° C. and at a flow rate of 41.0 g/minwherein the flow rate of methane was 18.4 g/min and the flow arte ofoxygen was 22.6 g/min. The gaseous reactants exothermically reacted toprovide a synthesis gas product stream. The first reaction zone wasmaintained at a pressure of 20 bara.

The product stream comprising hydrogen, carbon monoxide, carbon dioxide,and water was passed to the mixing zone at a flow rate of 40.99 g/minwherein the hydrogen flow rate was 4.30 g/min, the carbon monoxide flowrate was 29.32 g/min, the carbon dioxide flow rate was 4.61 g/min andthe water vapour flow rate was 2.76 g/min. The temperature of theproduct stream was 1200° C.

A second feed stream comprising ethane and oxygen was passed to themixing zone at a flow rate of 125.37 g/min wherein the ethane flow ratewas 100 g/min and the oxygen flow rate was 25.37 g/min. The temperatureof the second feed stream was 250° C.

The residence time in mixing channel was less than 5 ms.

The mixed feed stream with a combined temperature of 600° C. was passeddirectly to the second reaction zone wherein in the absence of acatalyst the oxygen was consumed and the ethane was converted toethylene. The second reaction zone was also maintained at a pressure of20 bara.

The product stream exiting the second reaction zone at a temperature of800° C. was cooled using a water quench to 600° C. within less than 20milliseconds of formation.

The % conversion of ethane was measured a 61.4% and the selectivitytowards ethylene was measured at 32.06%.

COMPARATIVE EXAMPLE

A mixed feed stream comprising hydrogen, oxygen and ethane was passed tothe mixing zone at a flow rate of 24.81 g/min wherein the hydrogen flowrate was 0.70 g/min the ethane flow rate was 18.53 g/min and the oxygenflow rate was 5.58 g/min. The temperature of the feed stream was 450° C.

The mixed feed stream was passed directly to reaction zone comprising asupported platinum group metal catalyst wherein oxygen was consumed andthe ethane was converted to ethylene. The reaction zone was also at apressure of 1.8 bara. The product stream exiting the reaction zone wascooled using a water quench to 600° C. within less than 50 millisecondsof formation.

The % conversion of ethane was measured a 67.6% and the selectivitytowards ethylene was measured at 79.0%.

It can be seen from the above examples that an autothermal crackingreaction can be operated in the absence of a catalyst and achieve a highhydrocarbon conversion and a high selectivity towards olefins.

1. A process for the production of olefins from a hydrocarbon saidprocess comprising the steps of: a) passing a first feed streamcomprising gaseous reactants to a first reaction zone wherein saidgaseous reactants react exothermically to provide a product stream b)producing a mixed feed stream comprising oxygen by passing the productstream produced in step (a) and a second feed stream comprising ahydrocarbon feedstock to a mixing zone and wherein oxygen is passed tothe mixing zone via one or more of (i) the second feed stream comprisinga hydrocarbon feedstock and (ii) a third stream comprising anoxygen-containing gas c) passing the mixed feed stream directly to anessentially adiabatic second reaction zone wherein in the absence of asupported platinum group metal catalyst in a second reaction zone thatdoes not contain any catalytic material that is capable of supportingcombustion beyond the normal fuel rich limit of flammability, at least apart of the oxygen is consumed and a stream comprising olefins isproduced d) cooling the stream comprising olefins exiting the secondreaction zone to less than 650° C. within less than 150 milliseconds offormation and wherein the temperature of the mixed stream is at least500° C., the mixing zone and the second reaction zone are maintained ata pressure of between 1.5-50 bar and the residence time within themixing zone is less than the autoignition delay for the mixed stream. 2.A process as claimed in claim 1 in which an additional feed streamcomprising hydrogen is passed to the mixing zone.
 3. A process asclaimed in claim 1 in which the residence time within the mixing zone isless than 100 milliseconds.
 4. A process as claimed in claim 3 in whichthe residence time within the mixing zone is less than 5 milliseconds.5. A process as claimed in claim 1 in which the reaction is carried outin the second reaction zone at a pressure of between 5 to 30 bara.
 6. Aprocess as claimed in claim 1 in which the second reaction zone does notcontain any material that would exhibit any substantial catalyticactivity.
 7. A process as claimed in claim 6 in which the secondreaction zone contains a stabiliser and/or packing material selectedfrom the group comprising porcelain, ceramics, alumina and silica thatdo not exhibit any substantial catalytic activity.
 8. A process asclaimed in claim 1 in which the second reactor contains an ignitionsource.
 9. A process as claimed in claim 1 in which the pressure of thesecond reaction zone is maintained at a pressure of between 5.0-10.0baraand the products are quenched by reducing the temperature to less than650° C. within less than 500 milliseconds of formation.
 10. A process asclaimed in claim 1 in which the pressure of the second reaction zone ismaintained at a pressure of between 10.0-20.0 bara and the products arequenched by reducing the temperature to less than 650° C. within 20milliseconds of formation.